If I intend to run an LED with a 5V source voltage and an LED forward voltage of 2V at 20ma it is my understanding that a 150 ohm resistor is suited.

I get this from using the formula V = IR rearranged to R = V/I. R = 3/0.02 A 3V voltage difference at 20ma.

This works in practice as well, the LED is well lit and I get the forward voltage and current I expect when measured.

How exactly are these concepts related? It is my understanding that at at V voltage with R resistance, a maximum of I current will be allowed through. Is my understanding of these concepts over simplified or valid?

  • \$\begingroup\$ "At V voltage" is inaccurate. \$\endgroup\$ – Ignacio Vazquez-Abrams Jun 12 '18 at 23:11
  • \$\begingroup\$ Could you elaborate? Is my concept of voltage misguided? \$\endgroup\$ – Zano Jun 12 '18 at 23:13
  • \$\begingroup\$ Note that if the LED has a maximum rating of 20 mA and you run it at 20 mA continuously, it will not have a long life. Either PWM it or run it at 50% to 75% of its max rating for a long life. \$\endgroup\$ – user105652 Jun 12 '18 at 23:14
  • \$\begingroup\$ From what you have written here your understanding appears reasonable and valid. I could be missing something but I don't see any logic flaws. \$\endgroup\$ – Redja Jun 12 '18 at 23:20
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    \$\begingroup\$ The "a maximum of" part is mistaken. Current, voltage, and resistance are precisely related - or one of the measurements is wrong or inapplicable. For example, while it works for the resistor, you can't really express the behavior of an LED as a simple resistance. Given that the forward voltage on an LED is not ultimately fixed, there are still a number of unknowns. \$\endgroup\$ – Chris Stratton Jun 12 '18 at 23:23

It's very important to remember that Ohm's Law applies to the voltage across a resistor and the current through the resistor.

If you know the source voltage and the LED forward voltage then you can use Kirchhoff's Voltage Law to determine what the voltage must be across the resistor. Then you apply Ohm's Law to determine what value of resistance will result in the desired current, now that you know what the resistor voltage must be.


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